After enucleation or evisceration of an eye, surgeons will routinely fill the void with an implant material to prevent the eye socket and surrounding tissue from collapsing and to provide support for a natural looking prosthesis. Over the years a wide variety of materials have been employed as orbital implants including gold, silver, vitallium, platinum, aluminum, cartilage, bone, fat, fascia, lata, sponge, wool, cork, rubber, silk , catgut, peat, agar, asbestos, ivory, paraffin and cellulose. More recently, surgeons have usually replaced the orbital contents of an enucleated eye with either glass, metal, silicone, methylmethacrylate, autogenous tissue or hydroxyapatite. Although each of these materials satisfactorily filled the orbit cavity and thereby supported the neighboring tissues, many of the materials available in the prior art had disadvantages. Prior art materials and methods designed to meet these objectives and criteria were plagued with problems including loss of eye movement, migration (implant drifting), extrusion (Implant rejection), poor movement of the prosthesis and drooping of the lower eyelid. Sterile hydroxyapatite, which can be obtained from coral or by synthetic means, has been a relatively successful material and was approved by the FDA for such surgeries in the 1980's.
After an eye is removed because of trauma, disease, pain, cosmesis or tumor, it is desirable to reconstruct the orbital area and replace the ocular area with a natural looking prosthesis to simulate the look of the eye. The custom external prosthesis, typically made from methylmethacrylate, are designed by ocularists who fit the prosthesis to the patient and decorate the external surface to realistically match the remaining healthy eye of the patient. In order for the prosthesis to appear realistic it is desirable to have the prosthesis track the movement of the remaining lateral or real eye. In this regard, some success has been achieved in attachment of the extraocular muscles which control movement of the eye to the implant which fills the orbit. After attachment of the muscles the orbital implant will mimic the motion of the remaining functional eye. In order to better translate the movement of the orbital implant to the prosthesis, the components can be coupled or linked together.
In addition to the motility function, the orbital implant supports the surrounding tissues and allows for placement of a natural looking prosthetic for the eye. One problem with prior ocular implants is that the eye socket would often appear to recede or sink back. The condition, characterized by an implant having a sunken appearance caused by the shrinking or sinking back of the eye socket, is referred to as enophthalmos. Efforts directed at mitigating enophthalmos of the anophthalmic socket have often resulted in limiting the motility of the implant and increasing pressure on the lower eyelid.
In connection with hydroxyapatite implants, one manner disclosed in the prior art to couple and secure the prosthesis to the orbital ocular implant involves providing the orbital implant with a post or projection on which the prosthesis is received by a complementary opposite depression or cavity provided thereon. The use of a ball and socket or peg coupling mechanism also helps to support the ocular prosthesis and thus can relieve pressure on the lower eyelid by supporting some of the weight of the prosthesis. In this regard some patients have been able to rectify a problem by a drooping lower lid using a direct coupling method. The post or peg is inserted into a bore hole which is drilled into the anterior surface of the orbital implant after it has been implanted and allowed to significantly vascularize. The post is retained within the bore hole by frictional engagement and eventually tissue grows along the sidewalls of the bore hole adjacent to the peg. The opposite end of the post, which extends outwardly from the anterior surface of the implant, is received in an opposite bore hole or cavity provided on the rear of the in the ocular prosthesis. The prior art also discloses inserting a cylinder having threads on the exterior surface of the cylinder and a smooth bore hole which can receive a peg or post. The prior art further discloses additional manners in which to couple implants with ocular prosthesis which are made from other materials. For example, some involve providing integral projections on either the orbital implant or the ocular implant which can be received by an opposite cavity and intended to be permanently secured. Others have taught the use of magnetic couplings between the prosthesis and the orbital implant. Due to excessive infection and extrusion rates, the use of such coupling systems have been, by in large, abandoned. The introduction of porous hydroxyapatite implants effectively mitigated many of the problems involving infection, extrusion and migration. The introduction of porous implants provided the opportunity for the ocular implant to allow for the ingrowth of vascular tissues into the porous surface of the implant. These materials have diffuse and small pores which allow the blood vessels and tissue to grow into them making them an integral part of the body. Vascularization of the implant minimizes the problems of migration and extrusion. Because non-porous implants have a higher incidence of failure due to infection and complications, porous implants are favored. Although hydroxyapatite is resistant to infection, under experimental conditions it has been established that the material interfered with normal host tissue response and led to chronic mild inflammation that did not completely resolve. Some additional drawbacks to the hydroxyapatite material are that it is abrasive, relatively heavy and must be carved from its natural state to conform to the shape and size of the orbital void. Furthermore, hydroxyapatite is relatively brittle and fragile and due to these inherent mechanical properties it was difficult to mechanically attach ocular muscles to the implant and provide a linkage to the prosthesis. The manners in which to couple the orbital implant to the prosthesis were limited to the post system described above.
Integration of the coupling link from the implant to prosthesis requires the patient to undergo a second operation after the orbital implant has been implanted, healed and become vascularized. During the second operation a hole is drilled at a location determined to align with the center of the eye. In order to minimize trauma to the conjunctiva which has grown over the implant surface, it is best to use a drill bit with the cutting portion limited to the ends of the bit and not up the sides is used. In the prior art, a post having a diameter of 2.5 mm was fitted within a 3 mm diameter hole. According to prior art methodology, a hole was drilled into the implant which required drilling into the brittle implant material to a depth of approximately 14 mm. This operation created drilling fragments and powder which was then removed. After the bore hole is formed, tissue is provided to line the interior confines of the bore hole. In the alternative, since the exterior region of the implant is invested with blood vessels and fibrous tissues which will support epithelial growth, epithelial tissue will grow down the sides of the drilled hole during a second healing process.
Although the results of this prior art procedures have generally been favorable, there have been reports of granulation tissue growth occurring within the central hole which pushes out the motility peg. Such growth, when it occurs, can be prolific requiring repeated excisions and repositioning of the peg. Because the motility peg is not secured to the implant, it is possible to lose the motility peg during cleaning of the prosthesis. Further, the method involves imposing significant additional trauma to the tissues which have grown into the orbital implant. In view of the complications which were raised, in some cases, integration of the prosthesis and implant was avoided.
Although the use of porous hydroxyapatite provided a number of advantages over the prior art materials used for orbital implants problems as described above continued to exist. During the 1980's porous plastic implants of a surgical grade polyethylene were developed which had a number of advantages over hydroxyapatite. These implants have superior strength, were light weight, and have proven to be effective in many of the applications which had been previously performed by hydroxyapatite materials. Porex Surgical of College Park, Ga. manufactures such implant materials under the trademark MEDPOR.RTM. and markets products designed for implantation into the anophthalmic socket identified as MEDPOR.RTM. spheres and the MEDPOR.RTM. CVA (conical volume augmentation) ocular implant. Porous polyethylene is an inert material which has the same advantages afforded by the porous surfaces provided by naturally occurring hydroxyapatite. The plastic is inert, stable and easily can be sterilized. Because the implant is synthetic, an uninterrupted supply of the material is readily available. Further, the material can be easily molded and shaped to appropriately fit an orbital void which requires an implant. Lastly, because the porous material is flexible and pliable, it enabled surgeons to employ new coupling methods between the implant and the ocular prosthesis and between the implant and the extraocular muscles. While hydroxyapatite may be brittle and can crack at the interface between a screw and the implant material, porous polyethylene can be compressed.